Following the War of the Gods, the Triumvirate created separate planes that embody different core beliefs, but prior to their war, there were only six different planes. The Protean Sea, is the plane on which all others exist within, it's the bending or movement across this space that allows transportation to the planes. There are the twin planes of Faerie and Shadow, the mirror planes of pure life and death energy. And finally, there's the Prime Material plane above, the twisting caves of the Dark Maze below, and Quintessence, the plane of elements, sandwiched between the two.
Added on by the gods following the sealing of the Concord are what's known as the Upper and Lower Planes. The Upper Planes tend to be more associated with positive concepts, these are the planes that the members of the Triumvirate made for themselves and their followers. Meanwhile, the Lower Planes are heavily associated with more negative concepts, these are the planes that the members of the Concord were imprisoned in.